The polyurethane elastic fiber has excellent stretching properties and is widely used in the fields of hosiery, underwear, sportswear etc.
Known processes for producing the polyurethane elastic fiber include a wet spinning method where a polyurethane solution is extruded and coagulated by passage through a coagulation bath, a dry spinning method where solvent is vaporized with hot air, or a melt spinning method where thermoplastic polyurethane is melted and extruded followed by solidification by cooling with air. Among these spinning processes, the melt spinning process is particularly advantageous in that organic solvent with strong possibility of polluting the human body and the environment is not used, so this process recently has attracted considerable attention as a spinning process which is not detrimental to the environment.
The melt spinning process is a process in which melted polyurethane is extruded through a spinning nozzle into air, solidified by cooling and wound as described above, so unlike the dry or wet spinning process, no volatiles are contained from the melting step to the cooling and solidification step. Accordingly, this melt spinning process is characterized in that the surface of the resulting fiber is frat and free of the uneven surface generated upon removal of volatiles from the inside of the fiber. Because of these characteristics, the polyurethane elastic fiber produced by the melt-spinning process is superior in wear resistance and further possesses the property of glistening.
However, relatively thin knitted goods such as stockings, tights, sox etc. have the disadvantage of too high glistening due to the above surface property of polyurethane elastic fiber. For example, black knitted goods generate glossy black luster. In stockings, tights, sox etc. made of covered fiber having nylon fiber etc. wound around the polyurethane elastic fiber, this luster phenomenon occurs very significantly due to relatively low degrees of coverage on the polyurethane elastic fiber.
To reduce the luster phenomenon, there is a method of increasing the number of twisting in the covering step in order to increase degrees of coverage. However, there is the disadvantage that the fiber is felt hard in proportion with an increase in the number of twisting for coverage. Further, there is also a method of dyeing the polyurethane elastic fiber darkly (e.g. black). However, the reduction in luster attained in this method is slight so significant improvements cannot be achieved.
There is also a known method of decreasing the luster phenomenon by roughening the surface of fiber. For example, there is a general method of roughening the surface of polyethylene terephthalate fiber by mixing inorganic fine particles with a polymer to form fiber and then dissolving and removing the surface of the fiber with a chemical such as alkali etc. to cause the inorganic fine particles to be removed therefrom so that the surface of the fiber is roughened.
Although this method is effective for polyethylene terephthalate fiber, it cannot be applied to polyurethane elastic fiber because there is no suitable chemical which can dissolve and remove the fiber surface.
Further, there is a method in which a large amount (e.g. about 30 to 40% by weight) of inorganic fine particles are previously mixed with a polyurethane polymer and melt-spun, and the surface of the resulting fiber is roughened in the step of solidifying the fiber by cooling. In this method, however, because a large amount of inorganic particles are contained in the polymer, the melt fluidity of the polymer is lowered, and in melt spinning, the polymer clogs a spinning nozzle, or fiber cutting frequently occurs to make spinning substantially infeasible. Even if spinning is feasible, the physical properties of fiber, such as strength, elongation etc. are significantly deteriorated.
In production of polyurethane elastic fiber by the dry spinning process, concave portions are generated after solvent is removed by heating for removal of volatiles. Further, there may occur cracking etc. in fiber by thermal deterioration, but there are a small number of concave portions, cracking is not significant, thus making the state of luster high. However, in polyurethane elastic fiber produced by the dry spinning process, upon being formed into knitted goods and then subjected to a dyeing step, a large number of concave portions and a large number of cracks are generated on the surface of the fiber because of removal of volatiles from the inside of the elastic fiber through the surface of the fiber to the outside, so the actual product has few problems resulting from the luster phenomenon.
However, knitted goods produced without undergoing a wet-heating step, for example tights etc. produced by previously dyeing nylon fiber as covering fiber, have high degrees of luster because volatiles in the inside of the polyurethane elastic fiber are not discharged to the outside.
Japanese Patent Publication No. 45684/1993 discloses a method of producing polyurethane elastic fiber by compounding aliphatic saturated dicarboxylic acid in an amount of 0.1 to 5 weight-% with polyurethane followed by dry spinning to produce polyurethane elastic fiber having a large number of uneven portions on the surface of the fiber. That is, this method is different from the present invention in that the aliphatic saturated dicarboxylic acid is compounded and the dry spinning method is used. The effect of the invention is also different between the present invention and this prior art method in that the former is directed to reduction in luster while the latter to improvements in stretching properties and traveling smoothness.
Further, the method described in the above-described patent publication is different from the present invention in that uneven portions on the surface of the fiber in the former are wavy (mountain range-like) while those in the latter are independent mountain-like protrusions. If the fiber is stretched for use, the uneven portions on the fiber surface disappear in the case of the wavy shape. On the other hand, the independent mountain-like protrusions such as those in the present invention maintain the uneven portions on the fiber surface. From this difference, the fiber of the present invention brings about significant reduction in the luster phenomenon. This difference in the effect is brought about by adopting the above constitution of the present invention.
A mixture of crystalline polyester based on polybutylene terephthalate and polyurethane is disclosed in Japanese Laid-Open Patent Publication Nos. 53448/1975, 50350/1977, 102365/1977, 9851/1978, 263457/1991, 275364/1992, 275365/1992, 313093/1994, 3135/1995 and 3136/1995 respectively. However, none of these publications disclose that the isocyanate group content in polyurethane is the range of the present invention.
Further, any of these publications are directed to molded articles which are not to be formed into fiber. Although the present inventor attempted to form these particles by spinning into fiber, fiber cutting was significant, thus making winding-up difficult or even if it could be wound, innumerable nodal defects occurred and adequate elongation could not be obtained. Further, mountain-like protrusions were observed on the surface of the wound fiber but the majority of them had a height exceeding 5.0 .mu.m to fail to achieve the effect of preventing luster.
The present invention is to provide polyurethane elastic fiber which is free of the luster phenomenon as well as a process for producing the same.
According to the process of the present invention, high-melting butylene terephthalate-based crystalline polyester (A) is first solidified and then stretched in draft and cooling steps where a melted polymer, discharged from a nozzle in a spinning step, is stretched in high draft and solidified. Hence, a large amount of mountain-like protrusions are generated on the surface of the fiber, and the polyurethane elastic fiber of the present invention can thereby be produced.